Gross Features: These aggressive tumors usually are large, with widespread and
diffuse destruction of the renal parenchyma, invasion of the renal pelvis and
perirenal fatty tissue and extensive vascular invasion. They are not sharply
demarcated and are of solid consistency. The gray tumors are often interspersed with
dark foci of softening and necrosis.
Microscopic Features: The neuroendocrine tumors of the kidney express a broad
spectrum of differentiation ranging from small cell carcinoma to well differentiated
classical carcinoid. Ultrastructurally, these tumors contain various amounts of
intracytoplasmic membrane-bound granules with dense cores ranging from 150-400 nm in
diameter. Silver stains (i.e. Bodian or Grimelius stains) have generally been
successful in staining these granules. In small cell carcinoma, the nuclei are
hyperchromatic and oval with abundant mitotic figures while the more differentiated
forms are have round nuclei with less anaplasia. Alternating broad trabeculae and
anastomosing cords embedded in a well vascularized stroma are the main characteristics
of the growth pattern in the carcinoid type.
Differential diagnosis: RCC unclassified, metastases.
